1. Field of the Invention:
This invention relates generally to nuclear reactors and more particularly to a core cooling system for a liquid cooled nuclear reactor having a cover gas above free surfaces of the reactor coolant.
2. Description of the Prior Art:
A nuclear reactor produces heat by fissioning of nuclear materials which are fabricated into fuel elements and assembled within a nuclear core. In commercial nuclear reactors, the heat produced thereby is used to generate electricity. Such nuclear reactors usually comprise one or more primary flow and heat transfer systems and a corresponding number of secondary flow and heat transfer systems to which conventional large steam turbines and electrical generators are coupled. Thus, a typical energy conversion process for a commercial nuclear reactor involves transfer of heat from the nuclear core to a primary coolant flow system, then to a secondary coolant flow system, and finally into steam from which electricity is generated.
In a liquid cooled nuclear reactor, such as a liquid metal cooled breeder reactor, a reactor coolant, such as liquid sodium, is circulated through the primary coolant flow system. A typical primary system comprises a nuclear core within a reactor vessel, a heat exchanger, a circulating pump and piping interconnecting the aforementioned apparatus. In nuclear reactors having more than one primary system, the nuclear core and the reactor vessel are common to each of the primary systems.
The heat generated by the nuclear core is removed by the reactor coolant which flows into the reactor vessel and through the nuclear core. The heated reactor coolant then exits from the reactor vessel and flows to the heat exchanger which transfers the heat to the secondary flow system associated therewith. The cooled reactor coolant exits from the heat exchanger and flows to a pump which again circulates the coolant into the pressure vessel, repeating the described flow cycle.
In the liquid metal nuclear reactor art, it is general practice to provide an inert gas blanket above free surfaces of the reactor coolant. This gas blanket, or cover gas, as it is more commonly referred to, prevents undesirable reactions of the liquid metal coolant with various reactor components. The cover gas above the level of reactor coolant within the reactor vessel prevents contact of intricate control rod drive mechanisms with the liquid metal reactor coolant. The circulating pumps of liquid metal cooled reactors also utilize a cover gas system. Here, the cover gas prevents contact of the pump motor and the pump seals with the liquid metal coolant. In nuclear reactors equipped with coolant reservoir tanks, a cover gas blanket is generally used above the level of coolant in this tank.
In accordance with the above, therefore, cover gas systems are generally beneficial; but they are not completely beneficial. For example, one disadvantage of the use of a cover gas system with a circulating pump involves the location of the pump within the primary system of the nuclear reactor. A so-called "cold leg" pump is one which is located between the outlet of the heat exchanger and the inlet of the reactor vessel. On the other hand, a "hot leg" pump is one which is located betweem the outlet of the reactor vessel and the inlet of the heat exchanger. Other than location, the primary difference between these two types of pumps is that the operating temperature environment of the hot leg pump is significantly higher than that of the cold leg pump. Obviously, from a design viewpoint, the cold leg pump is more desirable. However, the use of a cover gas, at least in the prior art, usually necessitated the use of a hot leg pump. This is because the vertical height of the pump cover gas space and therefore the length of the pump shaft must equal, as a minimum, the change in the level of reactor coolant within the pump from zero pump speed to operational speed if the same cover gas pressure is maintained over the pumps as over the reactor. For purposes of comparison, in one liquid metal system, the required cover gas height and therefore the length of the pump shaft was 12 feet for a hot leg pump and 36 feet for a cold leg pump. With this kind of dramatic difference, it is readily understandable why the prior art utilizes hot leg pumps, notwithstanding the associated disadvantages of the higher operational temperatures.
Nuclear reactors of the type described herein must and do consider reactor safety a foremost design requirement.
One area of reactor safety relates to the serious consequences of the "Loss of Coolant Accident" which could result from the rupture of one of the main coolant circulating lines, the most severe which is generally conceded to be a fully displaced "guillotine" rupture -- that is, a double ended rupture. Should the reactor core be deprived of coolant for an extended period of time, consequences as serious as a core meltdown could conceivably occur.
Liquid metal cooled reactors of the type considered herein have guard vessels surrounding the reactor vessel so that coolant cannot leak out by gravity. In addition, the pressure levels are so low that a rupture is extremely unlikely.
Upon the occurrence of a double ended rupture, which as noted above, is highly unlikely, the reactor will be scrammed and the coolant circulating pumps will be immediately shut down. However, during a period of time known as pump coast down, reactor coolant will be discharged by the pump out of both ends of the ruptured pipe. Depending upon the length of time of pump coast down, it is conceivable that a considerable amount of reactor coolant is discharged. The reactor vessel must therefore be designed to include a sufficient inventory of liquid metal above the core so that the amount of coolant discharged during the pump coastdown will not uncover the core.
The pressurized cover gas systems of the prior art tend to accentuate the discharge of reactor coolant from a double ended pipe rupture. A positive pressure at the inlet of the circulating pump will cause increased flow during pump coastdown. The cover gas pressure within the reactor vessel will cause more rapid discharge of the reactor coolant from the pressure vessel. Finally, the cover gas pressure within a reservoir tank will cause rapid depletion of reactor coolant which was stored for the purposes of providing emergency core cooling. Therefore, the cover gas systems of the prior art could have a detrimental effect should a double ended pipe rupture occur, however unlikely.